The American Society of Mechanical Engineers issues standards for mechanical and piping design. ASME standard B16.47 covers large diameter pipes up to a 60-inch bore. ASME standard B16.7 series A flanges are equivalent to MSS SP44 flanges. ASME standard B16.7 series B matches API 605 flanges.

B16.5 Pipe Flanges and Flange Fittings standard covers pressure-temperature ratings, materials, dimensions, tolerances,marking, testing, and methods of designating openings for pipe flanges and flanged fittings.The standard includes flanges with rating class designations 150, 300, 400, 600, 900, 1500, and 2500 in sizes NPS 1/2 through NPS 24, with requirements given in both metric and U.S units. The Standard is limited to flanges and flanged fittings made from cast or forged materials, and blind flanges and certain reducing flanges made from cast, forged, or plate materials. Also included in this Standard are requirements and recommendations regarding flange bolting, flange gaskets, and flange joints.

API Standards API flanges are designed to handle very high pressures and temperatures. API flanges have a smaller bolt circle than MSS flanges. All API flanges are ring joint flanges. API flange standards start with a number followed by a letter. API standard 17D is the specification for sub-sea well head flanges.

"The Differents Between API and ANSI Flanges" The difference between ASME/ANSI and API flanges is the fabrication material and the higher rated API operating pressure.
ASME/ANSI flanges are common in industrial process systems handling water, steam, air and gas. API flanges are manufactured for high strength operating refinery systems with products such as oil and explosive gases.
The flange standards API 6A and ASME/ANSI B16.5 are similar dimensionally - but the API 6A flanges are rated for higher pressures as shown in the table below:

API vs. ASME/ANSI Flanges

Flange

Pressure Class Rating (psi)

Nominal Size Range
(inches)

ASME/ANSI B16.5

API 6A

ASME/ANSI B16.5

API 6A1)

Weld neck

600

2000

1/2 - 24

113/16 - 11

900

3000

1500

5000

Blind and Threaded

600

2000

113/16 - 211/4

900

3000

113/16 - 203/4

1500

5000

113/16 - 11

1) In the old API standard, flanges ranged from 11/2 to 10(20) inches.

MSS Standards
MSS flange standards were adopted by American National Standards Institute. MSS flange standards start with an SP followed by a number. MSS SP-9 is the standard for spot facing flanges. MSS SP-44 is the standard for steel pipeline flanges. MSS SP-60 is the standard for connecting flange joint.

MSS SP-9 is the standard for spot facing flanges.
MSS SP-44 is the standard for steel pipeline flanges.
MSS SP-60 is the standard for connecting flange joint.

BS Standards
BS 10:1962 Specification for Flanges and Bolting for Pipes, Valves and Fittings. This covers plain, boss, integrally cast or forged, and welding neck type flanges, in ten tables. Although BS 10 is obsolescent, it remains in use for the dimensions of light duty, economy stainless steel flanges in applications where corrosion resistance and/or hygiene, rather than high pressures and temperatures, are the primary considerations. The following tables detail the applicable standard dimensions from Tables D, E, F and H of BS 10.